This invention relates to antennas.
More particularly, the present invention relates to integrated antennas used in portable communication systems.
An antenna is an essential element in most communication systems. This is particularly true for portable communication systems, such as cell phones, pagers, and laptop computers, where the size, weight, cost, and efficiency of the systems are critical design parameters. Types of antennas include monopole and dipole antennas, but these tend to be too large and obtrusive for the desired high operating frequencies, and, consequently, there is a need for elegant non-obtrusive antennas for portable communications systems.
FIG. 1 illustrates a plan view of an antenna 5 typically used in the prior art. Antenna 5 includes a voltage source 10 and a dipole antenna 20. Dipole antenna 20 has a length 25, a current distribution 30 and a radiation field pattern 40. For the antenna 5, most of the current is distributed within the middle section of the antenna and the ends do not radiate as effectively as the middle section. The two most important design parameters of antenna 5 is the electrical length, L, and the thickness parameter, t. The electrical length of a dipole antenna is given by       L    =                  λ        ⁢                  xe2x80x83                ⁢        ol            λ        ,
where l is the physical length of the antenna element, xcex is the resonant wavelength, and xcexo is the wavelength in free space. The thickness parameter of a dipole antenna is given by       t    =          a      l        ,
where 2a is the diameter or width of the dipole antenna.
FIG. 2 illustrates a capacitively loaded antenna 43. Since most of the current is closer to the center of antenna 5, it is possible to decrease the length of antenna 5 by capacitivly loading it without significantly distorting the current distribution. Capacitively loaded antenna 43 includes a voltage source 10 and a dipole antenna 50. Capacitively loaded dipole antenna 43 has a length 35, a current distribution 33, and a radiation field pattern 15. Further, the capacitive loading is provided by capacitors 45 which are electrically connected to dipole antenna 50. The result of the capacitive loading is to make length 35 less than length 25 while achieving the same resonance frequency. Also, current distribution 33 is more evenly distributed over the length of dipole antenna 50. A problem with capacitively loaded dipole antenna 43 is that the m resonance frequency is determined by length 35 and the values of capacitors 45. Once these parameters are set, the resonance frequency cannot be actively tuned.
A common type of antenna that is small and efficient for high frequency portable applications is the microstrip antenna. Microstrip antennas can be fabricated using inexpensive printed circuit board technology and can easily be integrated with other circuitry and electronic components. A patch antenna is a type of microstrip antenna that finds wide use in portable communication systems. However, most of the patch antennas in today""s communication devices have very limited tuning capability and a relatively large physical size. Therefore, it is desirable to have a small electronically tunable antenna for use in portable communication systems.
It would be highly advantageous, therefore, to remedy the foregoing and other deficiencies inherent in the prior art.
Accordingly, it is an object of the present invention to provide a new and improved electronically active antenna apparatus.
It is an object of the present invention to provide a new and improved electronically active antenna apparatus which has a small size.
It is another object of the present invention to provide a new and improved electronically active antenna apparatus which has an improved radiation efficiency.
It is another object of the present invention to provide a new and improved electronically active antenna apparatus which can be tuned over a wide range of frequencies.
A further object of the invention is to provide a new and improved electronically active antenna apparatus which is inexpensive to manufacture.
To achieve the objects and advantages specified above and others, an electrically active antenna apparatus is disclosed which includes a substrate, a RF feed positioned on the substrate, a radiator element positioned on the substrate and adjacent to the RF feed such that the radiator element and the RF feed are electromagnetically coupled, and a plurality of active devices that make electrical contact with the radiator element.
The antenna is actively tuned by incorporating a varactor, a negative differential resistance device, a resonant tunneling device, or micro-electro-mechanical system (MEMS) component, or combinations of these devices in the plurality of active devices. The integration of a negative differential resistance device reduces the antenna resistance and improves the efficiency of radiation. The plurality of active devices changes the capacitive loading and, consequently, the resonant frequency of the active antenna. The capacitance and the resistance of an active device can be tuned by applying a DC bias. The MEMS devices allow loading of the antennas with low loss capacitors which minimizes the power loss and increases the efficiency of the antenna. The magnitude of operational DC voltage applied to the MEMS in general is larger than the magnitude of the RF signal that is fed to the antenna. Therefore the capacitance of the MEMS devices will not be modulated by the RF signal and hence the harmonic signal generation will be minimized. Further, the placement of the active devices in relation to the antenna affects the resonant frequency and tuning characteristics. Thus, the physical size of the active antenna can be decreased and the resonant frequency can be tuned without significantly decreasing the effective resonant length.